This invention relates to an automotive vehicle rear wheel anti-lock brake control system.
When the brakes of a vehicle are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters including the road surface condition and the amount of slip between the wheel and the road surface. For a given road surface, the force between the wheel and the road surface increases with increasing slip values to a peak force occurring at a critical wheel slip value. As the value of wheel slip increases beyond the critical slip value, the force between the wheel and the road surface decreases. Stable braking results when the slip value is equal to or less than the critical slip value. However, when the slip value becomes greater than the critical slip value, the wheel rapidly begins to approach a wheel lockup condition resulting in a reduced vehicle stopping distance and a deterioration in the lateral stability of the vehicle.
Numerous wheel lock control systems have been proposed to prevent the wheels from locking while being braked. These systems generally prevent a wheel from locking by controlling the applied brake pressure when an incipient wheel lockup condition is sensed so as to limit wheel slip at or near the critical slip value and thereby establish substantially the maximum possible braking force between the tire and road surface.
Some of the known wheel lock control systems utilize an independent mode of braking wherein each of the front and rear vehicle wheels are individually controlled so as to establish the maximum possible braking force at each wheel during wheel lock controlled braking. By so maximizing the braking forces at each wheel, the stopping distance of the vehicle is minimized. However, under certain conditions, this mode of operation can lead to reduced vehicle stability. One such condition is a grossly different coefficient of friction between the right and left sides of the vehicle, hereafter referred to as a split coefficient of friction surface.
In order to improve vehicle stability during braking on surfaces having different coefficients of friction between the right and left sides of the vehicle, it has been proposed to control the two rear wheels with a common braking pressure in a manner such that a wheel lock condition is prevented even at the wheel which is on the side of the road surface having the lowest coefficient of friction. One known method for achieving this objective is to control the brake pressure at each of the rear wheel brakes in response to the conditions of the rear wheel being braked on the lower coefficient of friction surface. This form of control is commonly referred to as a select low mode of wheel lock control. The rear wheel being braked on the lowest coefficient of friction surface is typically indicated by the wheel having the lowest speed between the two rear wheels and a reference speed represented by the lowest speed of the two wheels is used as a basis for control.
However, when the rear wheels are being braked on a substantially uniform surface, the two rear wheel speeds are never exactly the same and are randomly higher or lower than each other. In this situation, the reference speed based on the lowest of the two wheel speeds is a noise intensified signal. Controlling in response to this noise intensified signal can result in a deterioration in the effectiveness of the anti-lock brake controller particularly where the control includes an anticipatory term.